Last updated: August 29, 2023

Summarytoggle arrow icon

Vaccination is a very effective measure for providing immunity to many infectious diseases. The discovery of vaccines played a central part in the eradication of smallpox and helped significantly reduce the incidence of potentially severe diseases such as poliomyelitis and measles. Live vaccines (attenuated, i.e. noninfective pathogens), inactivated vaccines (subunits or complete pathogens), viral vector vaccines, and nucleic acid vaccines (DNA, RNA, mRNA, or viral replicons) are used to achieve active immunization, which enables the host's immune system to build up a sustained immune response to specific pathogens. The immune response may be measured and quantified by assessing the antibody titer. In the event of potential disease (e.g., after exposure to high-risk pathogens), if the immune system is unable to produce sufficient antibodies fast enough, passive immunization can offer immediate short-term protection via direct injection of pooled antibodies for many conditions. Modern vaccines are usually well-tolerated, and adverse events are rare. However, the intervals between vaccine administration and possible contraindications must be considered.

For ACIP vaccination recommendations, see “Immunization schedule.”

Definitiontoggle arrow icon

  • Vaccine
    • A product (e.g., dead or weakened organism) that provides immunity from a disease
    • May be administered through injection, orally, or nasally
  • Vaccination: administration of a vaccine that induces an active immune reaction in form of cellular and/or humoral response, providing immunity against a pathogen
  • Immunization
    • The process by which a person becomes protected from a disease
    • Vaccines and recovering from some infections cause immunization.


Aims of routine immunizationtoggle arrow icon

  • Herd immunity
    • Once a certain percentage of the population has received immunization, non-vaccinated individuals (e.g., children too young to receive vaccination) will also be protected.
    • Mass vaccination: Vaccination of a large number of people in the shortest possible time after the outbreak of an epidemic, with the goal of herd immunity.
  • Eradication of disease
    • High immunization rates over prolonged periods of time can achieve eradication of certain diseases. [2]
    • To date, only two diseases have been eradicated by human efforts: smallpox (1980) and rinderpest (2011).
  • Lower incidence and associated risks: The Haemophilus influenzae type b (Hib) vaccine has decreased the number of cases of invasive Hib disease (e.g., pneumonia, bacteremia, meningitis, epiglottitis, infectious arthritis) in children younger than 5 by more than 99%. [3]

Passive immunizationtoggle arrow icon

Passive immunization Helps Beat The Disease Rapidly:” HBV, Botulinum, Tetanus, Diphtheria, and Rabies are indications for passive immunization.

Active immunizationtoggle arrow icon

Current vaccination recommendations for the US can be found in the “Immunization schedule.”

Live attenuated vaccinestoggle arrow icon

TYler And Paul Burnt their INFamous ROasted YELLOW-RUBy CHICKEN MEAt Very MUch”: TYphoid, Adenovirus, Polio, BCG, INFluenza, ROtavirus, YELLOW fever, RUBella, CHICKENpox, MEAsles, Varicella, and MUmps are live attenuated vaccines.


Inactivated vaccinestoggle arrow icon

Overview of inactivated vaccines
Whole vaccines Subunit vaccines
Protein-based Polysaccharide-based
  • Whole inactivated or dead pathogens (using chemicals or heat) that are unable to replicate
  • Surface epitopes remain unchanged, since they are important for triggering an adequate immune response.
  • Cause a weaker immune response, but are considered to be safer than live vaccines
Available vaccines
Mechanism of action
Special considerations
  • Not consistently immunogenic in infants

“Beware of Hepatitis A on your TRIP:” Hepatitis A, Typhoid fever, Rabies, Influenza, and Poliomyelitis.


Viral vector vaccinestoggle arrow icon

  • Definition: an unrelated virus (e.g., adenovirus, vesicular stomatitis virus, influenza virus) is modified to be used as a nonpathogenic vector that delivers genetic code to cells containing instructions for the production of the desired antigen. [11]
  • Mechanism of action
    • There are two types of viral vector vaccines: replicating and nonreplicating
      • Nonreplicating vector vaccines: enter the cells and induce the production of vaccine antigens, but cannot produce new viral particles
      • Replicating vector vaccines
        • Enter the cells, inducing the production of new viral particles
        • New viral particles go on to infect other host cells, leading to the production of further viral and vaccine antigens
    • Induce both humoral and cellular immune responses
  • Administration: usually injected intramuscularly, but can also be administered intranasally, intradermally, and orally
  • Available vaccines
  • Special considerations

Nucleic acid vaccinestoggle arrow icon

RNA vaccines [13][14]

  • Definition: a vaccine based on mRNA that delivers genetic code containing instructions for the production of the desired antigen to cells
  • Mechanism of action
  • Administration
    • Injected intramuscularly or intradermally
    • Require multiple doses
  • Available vaccines: COVID-19 vaccines (e.g., Comirnaty, Spikevax) contain modified mRNA embedded in lipid nanoparticles that encodes for the spike protein mRNA sequence of SARS-CoV-2
  • Special considerations
    • mRNA is a nonintegrating platform that is degraded by normal cellular processes. Due to its transitory nature, mRNA does not interact or integrate into the DNA and bears no risk of insertional mutagenesis. (e.g., COVID-19 vaccines are safe to use during pregnancy)
    • There is no potential risk of infection as mRNA is nonpathogenic.
    • Require strict cold-chain
    • Different techniques can be used to deliver the vaccine (e.g., injection of naked mRNA or encapsulated within nanoparticles or polyplex)

DNA vaccines [15]

  • Definition: A specific antigen-coding DNA sequence is introduced using a genetically engineered plasmid to induce endogenous antigen production in the host.
  • Mechanism of action
  • Administration
    • Intramuscularly or intradermally
    • Require multiple doses
  • Available vaccines: No DNA vaccines have been approved for human use in the United States.
  • Special considerations
    • Potential advantages observed in animal models are improved vaccine stability and the use of nonpathogenic agents.
    • In order to properly deliver the vaccine and ensure cellular uptake, injection needs to be followed by electroporation.

Vaccine administrationtoggle arrow icon

Adverse effects of vaccinationtoggle arrow icon

Common adverse effects [16][17][18]

  • Affects ∼ 1/3
  • Usually begin within the first 48–72 hours after administration and last 1–2 days
  • Symptoms
    • Local swelling, redness, and pain at the injection site
    • Low-grade fever (postvaccination fever)
    • Headaches
    • Fatigue
    • Flu-like symptoms
  • Live attenuated vaccine: can cause mild form of the disease, usually appearing within 1–3 weeks of administration; : usually caused by replication of the attenuated vaccine strain

Rare adverse effects [16][17][18]

There is no link between autism and vaccines or their ingredients. [20][21]

Contraindications for vaccinationtoggle arrow icon


Contraindications [23]

All children should be immunized with the standard doses of vaccines according to their chronological age; doses should not be adjusted to weight or height.

Pathogens affecting unvaccinated childrentoggle arrow icon

Pathogens affecting unvaccinated and underimmunized individuals
Disease Clinical features Treatment
Measles virus
Rubella virus
  • Symptomatic treatment
  • See “Treatment” in “Rubella.”
Varicella zoster virus
  • Prodromal phase: low-grade fever, malaise
  • Exanthem phase
    • Hundreds of severely pruritic lesions in varying stages of development (e.g., papules, vesicles, and crusted pustules)
    • Lesions first manifest centrally (i.e., on the face, scalp, and trunk) and spread to the extremities.
Mumps virus
  • Symptomatic treatment
  • See “Treatment” in “Mumps.”
Corynebacterium diphtheriae
Haemophilus influenzae type b
Streptococcus pneumoniae
Neisseria meningitidis
Clostridium tetani
Bordetella pertussis
Hepatitis A virus
Hepatitis B virus

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Referencestoggle arrow icon

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  2. Barrett JR, Belij-Rammerstorfer S, Dold C, et al. Phase 1/2 trial of SARS-CoV-2 vaccine ChAdOx1 nCoV-19 with a booster dose induces multifunctional antibody responses. Nat Med. 2020; 27 (2): p.279-288.doi: 10.1038/s41591-020-01179-4 . | Open in Read by QxMD
  3. Possible Side-Effects from Vaccines. Updated: December 2, 2016. Accessed: March 23, 2017.
  4. $Information Sheet: Observed Rate of Vaccine Reactions: Measles, Mumps and Rubella Vaccines.
  5. Offit PA, Bell LM. Vaccines. John Wiley & Sons ; 2003
  6. Fazlollahi A, Zahmatyar M, Noori M, et al. Cardiac complications following mRNA COVID‐19 vaccines: A systematic review of case reports and case series. Rev Med Virol. 2021.doi: 10.1002/rmv.2318 . | Open in Read by QxMD
  7. Hviid A, Hansen JV, Frisch M, Melbye M. Measles, Mumps, Rubella Vaccination and Autism. Ann Intern Med. 2019; 170 (8): p.513.doi: 10.7326/m18-2101 . | Open in Read by QxMD
  8. Vaccines Do Not Cause Autism. Updated: November 23, 2015. Accessed: March 23, 2017.
  9. Epidemiology and Prevention of Vaccine-Preventable Diseases - Pertussis. Updated: September 29, 2015. Accessed: March 19, 2017.
  10. Vaccine Recommendations and Guidelines of the ACIP - Contraindications and Precautions. Updated: January 30, 2017. Accessed: February 13, 2017.
  11. Rotavirus. Updated: November 15, 2016. Accessed: March 23, 2017.
  12. Opri R, Zanoni G, Caffarelli C, et al. True and false contraindications to vaccines. Allergol Immunopathol (Madr). 2018; 46 (1): p.99-104.doi: 10.1016/j.aller.2017.02.003 . | Open in Read by QxMD
  13. WHO - VACCINE SAFETY AND FALSE CONTRAINDICATIONS TO VACCINATION. Updated: January 1, 2017. Accessed: February 12, 2021.
  14. Leentvaar-Kuijpers A, Coutinho RA, Brulein V, Safary A. Simultaneous passive and active immunization against hepatitis A. Vaccine. 1992; 10: p.S138-S141.doi: 10.1016/0264-410x(92)90569-6 . | Open in Read by QxMD
  15. ACIP Vaccine Recommendations and Guidelines Timing and Spacing of Immunobiologics General Best Practice Guidelines for Immunization. Updated: April 7, 2023. Accessed: April 26, 2023.
  16. AAP Committee on Infectious Diseases. Red Book: 2021–2024 Report of the Committee on Infectious Diseases. American Academy of Pediatrics ; 2021
  17. Polio Elimination in the United States. Updated: October 25, 2019. Accessed: March 26, 2021.
  18. CDC - Haemophilus Influenzae Type b (Hib) VIS. Updated: October 29, 2019. Accessed: March 26, 2021.
  19. Park KS, Sun X, Aikins ME, Moon JJ. Non-viral COVID-19 vaccine delivery systems. Adv Drug Deliv Rev. 2021; 169: p.137-151.doi: 10.1016/j.addr.2020.12.008 . | Open in Read by QxMD
  20. Pardi N, Hogan MJ, Porter FW, Weissman D. mRNA vaccines — a new era in vaccinology. Nature Reviews Drug Discovery. 2018; 17 (4): p.261-279.doi: 10.1038/nrd.2017.243 . | Open in Read by QxMD
  21. DNA vaccines. Updated: January 1, 2021. Accessed: August 27, 2021.
  22. Vaccine Basics. Updated: March 23, 2017. Accessed: March 23, 2017.
  23. Songul Yalcin S, Engur Karasimav D, Yurdakok K. Measles vaccine failure in 9-month-old infants. Çocuk Enfeksiyon Dergisi/Journal of Pediatric Infection. 2016; 9 (4): p.153-160.doi: 10.5152/ced.2015.2195 . | Open in Read by QxMD
  24. Baicus A. History of polio vaccination. World J Virol. 2012; 1 (4): p.108.doi: 10.5501/wjv.v1.i4.108 . | Open in Read by QxMD
  25. Neonatal Immunology. Updated: January 1, 2018. Accessed: March 28, 2018.
  26. Foged C, Rades T, Perrie Y, Hook S. Subunit Vaccine Delivery. Springer ; 2014
  27. Epidemiology and Prevention of Vaccine-Preventable Diseases - Principles of Vaccination. Updated: November 15, 2016. Accessed: March 28, 2018.

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